Question

Обчислити суму 21.12

Answer 1

$21 ^{-1} +21 ^{-2} +21 ^{-3} =1/21+1/21^2+1/21^3=(21^2+21+1)/21^3=$$(441+21+1)/9261=0,05$

Answer 2

$21^{-1}+21^{-2}+21^{-3} \ k=21 \ k^{-1}+k^{-2}+k^{-3}= frac{1}{k}+ frac{1}{k^2}+ frac{1}{k^3}= frac{k^2+k+1}{k^3}= frac{21^2+21+1}{21^3}= \ = frac{441+22}{9261}= frac{465}{9261}=0.05$